The present invention relates to a method for laser hydrography in which at least one laser beam is emitted from a known location and at a known direction onto the surface of a body of water and through the water onto the bottom of the body of water, and measurement points of the water surface and the water bottom in a coordinate system are determined from transit-time measurements of the reflections of the laser beam on the water surface and water bottom.
The surveying of bodies of water by means of laser ranging measurements, so-called laser hydrography or—with a focus on measuring depths—laser bathymetry, is a method that has been established since the 1960s and that is used wherever the water depths are not too large and the water turbidity is not too high, so that reflections of the laser beam on the water bottom can be detected from outside of the body of water. The advantages of laser hydrography relative to underwater sonar or radar methods are the high data capture rate, large surface area coverage, and the possibility of simultaneously detecting areas of land at the edge of the water. A detailed overview of the basic principles and the prior art from the field of laser hydrography is contained in Guenther G. C., Cunningham A. G., LaRocque P. E., and Reid D. J., “Meeting the Accuracy Challange [sic; Challenge] in Airborne Lidar Bathymetry,” Proceedings of EARSeL-SIG-Workshop LIDAR, Dresden/FRG, Jun. 16-17, 2000.
All of the known laser hydrography methods work with large laser beam diameters, e.g., in the range of 2-5 m, on the water surface. The water surface can be assumed to be essentially flat and horizontal across such large beam diameters, because the effect of the wave motion of the water averages out across the beam cross section. Such a large laser beam diameter, however, greatly limits the measurement resolution and, indeed, both in the area resolution, i.e., the measurement-point density, and also—due to the averaging—in the time resolution of the transit time measurements and thus the range resolution. The known methods can therefore create only a relatively coarse grid of measurement points of the water surface and the water bottom with low range resolution.